Metallic aluminum is produced industrially by electrolysis of alumina in solution in an electrolytic bath primarily made up of cryolite, using the Hall-Héroult process. The electrolytic bath is contained in a pot of an electrolytic cell, comprising a steel shell coated on the inside with refractory and/or insulating materials, at the bottom of which a cathode assembly is located.
Anodes, typically made of carbonaceous material, are partially immersed in the electrolytic bath. Each anode is provided with a metal stem designed to connect it electrically and mechanically to an anode frame that is mobile in relation to a gantry fixed above the electrolytic cell.
A plant for the production of aluminum includes a great number of cells, typically one or more hundred, aligned along an axis. An electrical connection device including an array of electrical conductors connects the cathode assembly of cell (N−1) in series to the anode frame of cell (N) located immediately downstream, in the direction of current flow. The ends of the conductors, at the beginning and end of the series of cells, are connected to the positive and negative outputs of an electrical sub-station for rectification and regulation.
The current passing through the successive cells is very high, typically about 200,000 to 500,000 A. Because of this, the array of electrical conductors is designed so that the effects of the large magnetic fields generated compensate each other, so that the problems caused by these magnetic fields (bending of the upper surface of the molten metal in the pot, instabilities, etc.) are reduced.
Because of wear caused by the operation of a cell (N), the pot must be periodically repaired or replaced. In order for the other cells of the series to continue to produce, the cell (N) under consideration is bypassed, so that the current can pass directly from cell (N−1) to cell (N+1), for the time it takes to replace the pot of cell (N).
For this purpose, the practice of placing short-circuiting wedges between a first conductor, connected to the cathode assembly of cell (N−1), and a second conductor, connected to the cathode assembly of cell (N), is known. Because of this the current flows from the cathode assembly of cell (N−1) to the cathode assembly of cell (N), without going through the anode frame of cell (N), and is then sent to the anode frame of cell (N+1).
Because of the very high current flowing through the conductors, it is generally necessary to use at least two wedges in parallel, so that each wedge receives only part of the total current running through the conductors.
The problem encountered is that the layout of the conductors is restricted for reasons of magnetic field compensation, as indicated above, but also of spatial requirements.
One therefore generally has a conductor layout in which:                the first conductor has a portion located between said pots (N−1) and (N) in which the current flows towards the alignment axis of the pots;        the second conductor has a portion located between said pots (N−1) and (N) and in which the current flows away from the alignment axis of the pots;        
said portions of the first and second conductors being substantially parallel with each other.
In order to bypass cell (N), a first wedge and a second wedge are interposed between said portions of the first and second conductors, the second wedge being located more towards the alignment axis of the cells. Because of this, two paths of current flow from the first conductor to the second conductor are created, namely a first path through the first wedge and a second path through the second wedge. Due to the opposite direction of flow in the first and second conductors, the two paths have different lengths. Specifically, the second path is longer than the first, and therefore has a higher electrical resistance (due to the similarity of components, i.e. the wedges and conductors).
The result is a significant imbalance between the currents flowing through the wedges. For example, the first wedge may have up to 70% of the total current, and the second wedge only 30%. This is not desirable. On the one hand, the first wedge may deteriorate prematurely. On the other hand, the current imbalance may lead to a limitation of the current in the first wedge, and under-utilization of current capacity in the second wedge, this thereby limiting the overall current capability of the bypassing assembly.